[jasonmunro]

The liquidity comes from the HFT market maker's own money. Market makers are required to post bid and offers of a reasonable level of liquidity in all symbols they are registered to "make markets" in. It may seem like a simple skim off the top operation, but it's not that simple. If they do not maintain their bids/offers, they are disqualified from market making.

If they do maintain them, they reap a number of benefits including discounted pricing from trading platforms and IIRC, the ability to do naked short selling (which IMO should be off-limits for all trading firms). I'm not defending HFT here, just stating a fact that market makers are a part of the trading ecosystem.

Keep in mind that "making" and "taking" liquidity is not the same thing as buying and selling. Market makers are required to post both bids and offers. The difference is that market makers put their orders (buy or sell) on the "book", which means they are offering liquidity in both directions. The order that comes in to match (think "market" order to buy or sell) is the "taker". Firms that supply liquidity are rewarded by trading platforms (unsurprisingly, since those firms "make" their market), and firms that match those orders (takers), are charged for the service.

Not all HFT outfits are "market makers", but many are. I can't say specifically if Virtu is a market maker, even if I remembered :) The point is that liquidity providers don't pay for their trades, they are paid for them, so it's a natural fit for a smart HFT operation.

[consz]

> which IMO should be off-limits for all trading firms

Out of curiosity -- why?